It depends on what you mean by “know”. I would expect one to be aware of the concept and be able to understand it and use it if needed. The question is whether being able to explain differences between mutex and semaphore on the spot is correlated with good senior performance.

Similarly, I would expect an engineer to be able to solve a quadratic equation, but I don’t believe remembering the right formula on the spot is a good predictor of job performance.

Btw, to contrast this with a good senior interview even further, I wasn’t asked anything meaningful about software architecture…